As I understand it, water purification tablets (Chlorine, in this case) are just chemical tablets sealed in foil. So what causes them to expire? Are they still safe to use after their expiration date? That is, will they be dangerous or will they just not be as effective?

2 Answers
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There are two common chemical options to water purification; chlorine and iodine. Both forms use some kind of expiration date.

The consensed quantity of the chemical slowly degrades. Some things like sunlight cause the degradation process to speed up, so these are usually packaged in foil (as in your case) or brown glass bottles. Different recipes and forms cause different degredation speeds, the people who have studied the numbers published the expiration date you are looking at.

The expiration date is usually a safe date for the tablet when stored properly (temperature, moisture, sunlight, etc). That means you can probably go a few months past the date and not worry about anything. theoretically you could use larger quantities of tablets that are much older but that's dangerous territory and not very weight conscious. The reality is the tablets are cheap enough that whenever possible you should buy new.

This is the kind of thing where the only people who play around with clean water are the ones who have never experienced the effects of dangerously unclean water in one way or another.